Hydraulic valve-lash-adjusting elements serve to adjust the lash which forms due to wear or thermal expansion between the transmission elements of the cam lift and the gas-exchange valves of an internal combustion engine. This is intended to achieve a low-noise and low-wear valve train and the greatest possible conformity of cam rise curve and valve travel curve.
A hydraulic valve-lash-adjusting element for the valve train of an internal combustion engine is disclosed in EP 1 298 287 A2 and is characterized by the following features:                a housing of a valve-lash-adjusting element has a blind bore in which a plunger is guided with sealing clearance;        the plunger has a plunger head, which together with the blind bore defines a high-pressure space, whereas a low-pressure space is located above the plunger head;        the pressure spaces are connected by a central axial opening in the plunger head, this axial opening being controlled by a control valve arranged on the underside of the plunger head;        the control valve has a valve-closing body which has a hemispherical sealing surface, which together with a cylinder-ring-shaped sealing surface arranged on the plunger head forms a circular sealing line;        a valve-body spring, which is supported on a step between the central axial bore and a coaxial bore located below the latter, loads the valve-closing body in the opening direction, whereas positive pressure in the high-pressure space loads the valve-closing body in the closing direction;        a closing-body cap has a cap flange, an essentially cylindrical cap centre piece and a cap base provided with at least one opening;        the closing-body cap is supported and centred on the plunger head and serves as a travel limit for the valve-closing body;        pressure is applied to the plunger via the cap flange by a compression spring arranged in the high-pressure space.        
Such hydraulic valve-lash-adjusting elements, on account of the reversed arrangement of the closing-body springs compared with conventional hydraulic valve-lash-adjusting elements, are called reverse spring hydraulic valve-lash-adjusting elements and are abbreviated to RSHLA for the description below.
RSHLAs are distinguished by a positive effect on thermodynamics, pollutant emissions and mechanical stressing of the internal combustion engine. An important precondition for satisfactory functioning of the RSHLAS is constant travel and closing pressure of the valve-closing bodies. Constant idle travel of the RSHLAs is achieved as a result.
The valve-body spring of EP 1 298 287 A2 establishing the generics type is designed in such a way that the valve permits a fluid transfer between the high-pressure and low-pressure spaces during assembly of the RSHLA but closes as quickly as possible against the spring force of the valve-body spring when pressure increases in the high-pressure space. This spring force must therefore be relatively low. Therefore the spherical valve-closing body can be set in rotation by a possible lateral incident flow and can be laterally displaced with the valve-body spring. As a result, the closing force of the valve-body spring and consequently the idle travel of the RSHLA are changed. In the extreme case, the valve-body spring may pass into the seating gap of the control valve, which may lead to further variations in the idle travel and to the detuning or even to the total failure of the RSHLA and to the destruction of the valve-body spring.